1. Field of the Invention
This invention relates to an ultrasonic signal generator making use of a piezo electric transducer.
2. Prior Art
Such a signal generator is used for example in a flaw detector. The piezo electric transducer is connected to a pulse generator and it is well known to use, as the generator, a capacitor and a switch which discharges the capacitor into the load formed by the transducer. Such an arrangement is shown, for example, in French Patent Specification No. 1525998. The generator must include means by which reflected signals picked up by the transducer and converted thereby to electric signals are fed to a receiver. It is commonly necessary for the measuring instrumentation to be remote from the point to be examined and thus heretofore it has often been necessary to feed the transducer through a relatively long cable from the generator. Although for many purposes, a long cable can be successfully used, the cable losses limit the length which can be used while still obtaining sufficient power to be applied to the transducer. The greater the cable length, the larger the power required and hence the need for a bulky and expensive generator. Moreover signal distortions arise due to reflections at the mismatches introduced by the cable. As is well known, the mismatch from the generator to the cable is mostly due to the variation of the negative resistance of the switch and to the nonlinearity of the other components which arise because of the high voltage and current stresses created during the pulse generation. At the transducer end of the cable, there is a mismatch between the cable and the transducer which is time dependent; at the beginning of the pulse the transducer acts as a capacitor because its electrodes are stationary and at this stage, therefore all the incident power is reflected back through the cable to the generator with an apparent source impedance of about zero ohms and a phase shift of about 180.degree. depending on the rate of the rise of the voltage. The losses are resistive if no coil or transformer is used (which would reduce the crystal damping factor, restrict the band pass and might introduce a parasitic secondary oscillation).